(Apologies in advance for giving so few details below, but trust me that the set-up and the problem I'm dealing with are truly baroque, and even a superficial description would result in a far lengthier and complicated post than any of you would want to read!)

I'm trying to back up a disk that has been acting very strangely lately, but my back-up attempts fail after a few minutes, because the disk's file system (at /dev/disk4) "spontaneously" unmounts. (I doubt this unmounting is literally "spontaneous", but to my ignorance it looks that way.)

I would like to "lock" the mount, so to speak, so that no other process can unmount it (other than something like umount /dev/disk4issued by me). Is there a way to achieve such a lock?

(The flavor of Unix I'm dealing with happens to be OS X, but I am interested in, say Linux-specific solutions too, since they may provide useful keyword for further Googling. Also solutions for any other BSD-ish flavor of Unix would be welcome.)

(It goes without saying that I am interested only in locking schemes that do not prevent me from backing up the disk!)